1. Field of the Invention
This invention relates to an image forming apparatus such as a printer, photocopier, and a facsimile machine and, more particularly, to an apparatus utilizing an intermediate transfer body.
2. Description of Related Art
As an image forming apparatus of a full color electrophotographic type, an apparatus has been known to form full color images by sequentially forming plural toner images on a single photosensitive body and overlapping those toner images on an intermediate transfer body. More specifically, FIG. 10 illustrates the above structure. Respective toner images are sequentially formed upon sequentially facing developing units 4a, 4b, 4c, 4d having toners in different colors onto a first image carrier 1 as a sole photosensitive drum by means of a developing rotary apparatus 5. The respective color toner images are overlapped sequentially onto an intermediate transfer body 61 from the image carrier 1, and the toner images overlapped on the intermediate transfer body are transferred to a transfer material P by a secondary transfer roller 66. The transfer material carrying the toner images is fed through a fixing apparatus 8, thereby actually utilizing an image forming apparatus in which permanently fixed images are obtained in application of heat and pressure in the fixing apparatus. To form a full color image, image formation is made by overlapping four times the toner images on the intermediate transfer body where the toners in four colors are used.
The image forming apparatus above described does not have high productivity because the intermediate transfer body is required to be rotated the same times as the color number of the overlapped toners to overlap the toner images. Another image forming apparatus having plural photosensitive bodies, capable of overlapping the toner images in the prescribed number of colors on the intermediate transfer body where the intermediate transfer body is rotated twice has been known as an image forming apparatus improving the productivity but not rendering the apparatus itself significantly larger. This apparatus is called as a two-path system. In a case where, e.g., images overlapped with four color images are formed, such an image forming apparatus has two photosensitive bodies and forms a toner image overlapped with the four color toners upon rotating the intermediate transfer body two turns as, at every turn of the intermediate transfer body, the toner images of two colors are carried on the intermediate transfer body. That is, in this situation, there is an advantage to ensure a double productivity with respect to the image forming apparatus having a single photosensitive body.
However, the intermediate transfer body is required to be turned twice for image formation, and as a general rule it is necessary to escape members otherwise contacting to the toners on the intermediate transfer body so that the toner images on the intermediate transfer body are not disturbed between the transfer to the intermediate transfer body at the first turn and the transfer at the second turn. That is, in respect to the members contacting to the intermediate transfer body, attaching and detaching operations are needed during the image forming process, so that vibrations due to attaching and detaching operations may raise a problem rendering latent image formation disordered.